Where a synecdoche is a specific type of metonym, a merism is a specific type of synecdoche in which a phrase refers to something by the name of a few of its components (usually two in contrast). Like other metonyms, their usage most often comprises pre-established terms (such as saying “high and low” or “near and far” to mean “everywhere“), rather than existing in unique cases.
The slog can corrode you, mind and body. [Referring to the ‘whole of a person’ to mean completely]
Don’t let the slog waste your blood, sweat, and tears. [Referring to products of ‘bodily exertion’ to mean hard work]
Being the worst is the slog‘s bread and butter. [Referring to ‘basic needs’ (by way of food) as a function of their acquisition to mean manner of supporting oneself]
Aside from being an uglier word to say than look at, a synecdoche is a specific type of metonym where something is referred to by either a component of itself or, conversely, a broader category to which it belongs. When I call myself a writer, I’m naming one aspect of the process to say that I’m a storyteller.
I need to find a way to put the slog in irons. [Referring to shackles by their material]
Seeking to escape the slog, we entreated the Church to grant us asylum. [Referring to specific people by the organization they belong to, and that organization by the building it works in]